<meta charset="utf-8">
<script>
    max = 2//可以多调用两次
    function changeNum(a, b) {
        max--
        function f(c) {
            return '第' + c + '个值的结果是 '
        }
        alert(f('一') + b + f('二') + a)
        if (max == 0)
            changeNum = null
    }
    changeNum(1, 2)
    changeNum(2, 1)
    //4.可以（最）多调用两次
    changeNum(3, 4)
</script>